Comprehensive Overview of Lipid Metabolism: Digestion, Absorption, and Fatty Acid Oxidation

1. Chapter 8 Metabolism of lipid Cai danzhao

2. Lipids are substances that are insoluble in water but soluble in organic solvents. • Including: Fats (triglycerides，triacylglycerols，TAG) Function: store and supply energy phospholipids Lipoids cholesterol and cholesterol ester glycolipids Function: as membrane compounds

3. Cholesterol (ch) Fatty acid Triglyceride(TAG) Phospholipid(PL) Cholesterol ester (CE)

4. Carboxyl Hydrocarbon chain unsaturated FA (one double bonds) Saturated FA

5. Nomenclature: the chain length number of double bonds Palmitic acid 16:0 Oleic acid 18:1(Δ9)

6. Section 8.1 Digestion and Absorption of Lipids

7. Digestion of Lipids • Location: duodenum, small intestine • Condition: 1. bile salts (emulsification) 2. lipolytic enzymes Pancreatic lipase Phospholipase A2 Cholesterol esterase

8. Process bile salts food lipids small particles pancreatic lipase triglyceride 2-monoacylglycerol + 2FFA phospholipase A2 phospholipid lysophosphatide + FFA cholesterol esterase cholesterol ester cholesterol + FFA

9. Pancreatic colipase • Pancreatic colipse is the necessary cofactor of pancreatic lipase. • Excreted by pancreatic acinar cells as a proenzyme, which activated in the small intestine. • Can anchor the lipase to the surface of lipid micelles. • Assist lipase in two ways: 1.enhances the lipase activity 2.against the inhibitory effects of bile salts and surface denaturation.

10. Absorption of lipids • Medium and short chain fatty acid (10Cs or less) TAG emulsification absorption intestine mucosal cells degradation FFA and glycerol transport portal vein blood circulation

11. Long chain fatty acids (12-26C) + monoacylglycerol absorbed synthesis epithelial cells TAG + lipoproteins • lysophosphatide + FFA CM absorbed synthesis (chylomicron) epithelial cells PL • cholesterol + FFA lymphatic system absorbed synthesis epithelial cells CE blood circulation

12. Acyl CoA synthetase CoA + RCOOH RCOCoA ATP AMP PPi Acyl CoA transferase Acyl CoA transferase R2COCoA CoA R3COCoA CoA TAG synthesis in epithelial cells:monoacylglycerolpathway monoacylglycerol 1,2-diacylglycerol TAG

13. Section 8.2 Metabolism of Triacylglycerols

14. Degradation of TAG glycerol FA

15. Lipolysis: also named fat mobilization, is a process breaking down the fat (TAG) stored in adipose tissue and liberating the glycerol and FFAs from which into the blood circulation. • Key enzyme: TAG lipase (hormone sensitive lipase, HSL)

16. Lipolytic hormones: stimulate TAG hydrolysis Glucagons, Adrenocorticotropic hormone (ATCH) epinephrine norepinephrine • Anti-lipolytic hormones: stimulate TAG formation insulin prostaglandin E2 nicotinic acid

17. H2O FA H2O FA H2O FA TAGDAG MAG Glycerol TAG lipase a DAG lipase MAG lipase TAG lipase a ADP + Pi TAG lipase b ATP cAMP dependent protein kinase ATP cAMP 5’-AMP Adenylate cyclase Phosphodiesterase  lipolytic hormones (Epinephrine ) anti-lipolytic hormones (insulin)

18. FFA +plasma albumins fatty acid- transport albumin complexes all the body glycerokinase • Glycerol glycerol-3-phosphate dihydroxyacetone phosphate glucose metabolism Notation: adipose cells and skeletal muscles lack glycerokinase, can not use glycerol well

19. β-Oxidation of Fatty acids • FAs are the major energy source of human the biologically available energy in TAGs: ~ 95 % in their 3 long-chain FAs ~ 5% in their glycerol Oxidation location: in the cytoplasm and mitochondria of most body cells (except those of the brain and intestine)

20. The process of FA degradation • Activation • Transport into mitochondria • β-oxidation • Acetyl CoA utilization into citric acid cycle change to ketone bodies into other metabolic pathway

21. + CoA-SH 1.Activation of FA Activation of FA takes place on the outer mitochondrial membrane (in cytoplasm) Acyl CoA synthase Acyl-CoA Fatty acid ATP AMP+PPi

22. 2.Transport of Acyl CoA into mitochondria key enzyme: carnitine acyltransferase I translocase

23. 3. β-oxidation of Acyl CoA • In mitochondrial matrix,successive 2-Cunits are removed from the carboxyl end ofthe fatty acyl chain in the form of acetyl-CoAby a repeated sequence of 4 reactions, and the oxidation process take place at the β-carbon.

24. To Palmitoyl CoA (C16): 1.Dehydrogenation (FAD) 2.Hydration 3.Dehydrogenation (NAD+)

25. 4.Thiolysis Result of a round of β-oxidation: 1NADH, 1FADH2, 1Acytyl CoA, 1Acyl CoA (Cn-2)

26. 1 2 3 4 5 6 7 C16 Acetyl -CoA Acyl CoA (Cn-2) can now go through anotherset of β-oxidationreactions

27. 1 molecule of palmitoyl-CoA will pass through the sequence 7 times, eventually be oxidized to: 8 Acetyl CoAs 7 NADHs 7 FADH2s

28. ATP produced during oxidation of palmitate 2.67 Glucose (C16): 85.3 ATP

29. Alternative Oxidation Pathway of Fatty Acids oleic acid (18:1, Δ9): Can produce a cis-Δ3 C12 acyl CoA, but β-oxidationacts only on trans double bonds. • 1.Unsaturated FA Enoyl-CoA isomerase: make a trans- Δ2 C12 acyl CoA, then β-oxidationcancontinue

30. 2,4-dienoyl-CoA reductase Can convert cis- Δ4double bond to trans- Δ3,

31. 2.Peroxisomal Fatty Acid Oxidation • for very long chain FA digestion.(C20、C22) • No ATP produced. • FA reduced in length by this pathway will be transferred to mitochondria for further oxidation. very long chain FA(C20、C22) （peroxisomal） （mitochondria） Acyl-CoA oxidase （FAD） chain shorted FA βOxidation

32. 3.Propionyl CoA • β-oxidation of the odd-chain fatty acids, which are relatively rare in nature, produce a propionyl CoA in the final round. carboxylase Succinyl CoA CH3CH2CO~CoA Citric acid cycle

33. Ketone Bodies Formation and Utilization • Ketone Bodies are acetoacetate (30%) β-hydroxybutyrate (70%) acetone • Generated in liver cells (mitochondria), used by extrahepatic tissues (mitochondria also). • Precursor: Acetyl CoA

34. CO2 Ketogenesis HMGCoA synthase Acetoacetyl CoA CoASH CoASH 3-hydroxy-3-methylglutaryl CoA NADH+H+ NAD+ β-hydroxybutyrate acetoacetate acetone

35. Utilization of ketone bodies (cardiac, kidney, brain, skeletal muscles) β-hydroxybutyrate NAD+ NADH+H+ Succinyl CoA CoASH+ATP acetoacetate succinate PPi+AMP Acetoacetyl CoA CoASH

36. Physiological significance of ketogenesis • A way by which liver transfer fuel to extrahepatic tissues (prolonged starvation), ketone bodies can replace glucose as the major source of energy, especially for brain. • The normal concentration of ketone bodies in blood is very low. ﹤0.5mmol/L

37. Under starveling condition, ketogenesis is accelerated. • Under some pathological condition (such as diabetes), the synthesis is faster than utilization, so the concentration of ketone bodies in the blood is high, (up to 20mmol/L), which is called ketonemia , • ifthe concentration is too high to be excreted in the urine, that is ketonuria.

38. Ketone bodies are acidic compounds, the accumulate of which in the blood will decreasethe pHof blood , causeketoacidosis.

39. Regulation of ketogenesis • 1.Feeding status: hungry state: lipolytic hormones (glucagon) FA oxidation ketogenesis lipolysis FFA Feeding state FA oxidation ketogenesis insulin lipolysis FFA

40. 2.Metabolism of glycogen in the hepatic cells Sufficient glucose supply: FFA triacylglycerols Glucose deficiency: β- Oxidation ketogenesis FFA

41. 3.Malonyl CoA concentration Malonyl CoA can inhibit carnitine acyltransferase Ⅰ. malonyl CoA transportion of fatty acids into mitochondria β- Oxidation and ketogenesis

42. 8.2.2 FA Biosynthesis • FA synthesis is not the reverse of degradation: different pathways,enzymes, location of cells • Location of FA synthesis: cytoplasm of liver (major), adipose and other tissue cells. • First step: synthesis of palmitic acid

43. Palmitic Acid Biosynthesis Material: • acetyl CoA (come mostly from glucose) • NADPH (pentose phosphate pathway or produced by malate enzyme.) • ATP • HCO3-

44. ATP-citrate lyase MITOCHONDRIA CYTOSOL Acetyl CoA • Acetyl CoA must be transport to cytosol. ATP Acetyl CoA citrate citrate Citrate synthase oxaloacetate oxaloacetate malate pyruvate pyruvate citrate pyruvate cycle Inner membrane

45. Formation of malonyl CoA ATP + Acetyl CoA + HCO2-Malonyl CoA + ADP + Pi acetyl CoA carboxylase biotinact as CO2 carrier

46. Acetyl CoA carboxylaseis the key enzyme of the FA synthesis. • Allosteric regulation: up-regulate: citrate and isocitrate down-regulate: palmitoyl CoA • Phosphorylation regulation up-regulate: dephosphorylation (insulin) down-regulate: phosphorylation (glucogen)

47. Repetivity steps catalyzed by Fatty Acid Synthase CH3COSCoA + 7 HOOCH2COSCoA + 14NADPH+H+ CH3(CH2)14COOH + 7 CO2+ 6H2O + 8HSCoA+ 14NADP+ • The chain of FA grows 2-carbons per cycle. • The reactions are similar to the reversal of FA β-oxidation.

48. Fatty Acid Synthase In E.coli (becteria): Fatty acid synthase system (TypeⅡ system) contain 7 enzymes organized into a cluster. • In mammalian: Fatty acid synthase (Type Ⅰsynthase) is a single multifunctional polypeptide with 7 activities.

49. The acyl carrier protein (ACP) carries a growing fatty acyl chain from one active site to the next.

50. Process : 1.Charging β-ketoacyl-ACP synthase (KS) with an acetyl group